1. Field of the Invention
The present invention relates to a seal for a regenerative heat exchanging system. More particularly, the present invention relates to an improved seal that is used to prevent bypass leakage of hot gas or cold air through a heat exchanging system.
2. Description of the Related Art
Regenerative heat exchangers are used with many types of machinery that exhaust hot gas and operate more efficiently when supplied with preheated air. These types of machinery include power plants, chemical processors, refineries, pulp and paper mills, and ship engines. Typically, two fluid stream passages extend through a heat exchanger. The first passage is an exhaust or hot gas conduit that communicates with a hot exhaust outlet of the machinery. Hot exhaust gases flow from the machinery exhaust into the hot gas conduit to the heat exchanger. The second passage is an intake or cold air conduit that communicates with a cold air intake passage of the machinery. The cold air conduit feeds air into the intake passage of the machinery. As is known in the art, the regenerative heat exchanger extracts heat from the exhaust gases of the machinery and transfers the heat to the cool air conduit, so that the machinery is supplied with heated intake air which improves the operating efficiency of the machinery.
One typical heat exchanger includes a movable heat exchanging body that moves between the hot gas conduit and the cool air conduit. The movable heat exchanging body cyclically collects heat from the exhaust conduit and transfers this heat energy to the intake conduit. In this manner, the heat from the machinery exhaust is used to warm the air that is being supplied via the intake conduit to the machinery. By supplying the machinery with preheated air, the efficiency of the machinery is improved. Additionally, the heat exchanger is environmentally friendly as it recycles heat that would otherwise be exhausted into the earth's atmosphere.
This type of heat exchanger is referred to as a Ljungstrom.TM.-style preheater. The heat exchanging body in a Ljungstrom-style preheater is typically cylindrical in shape and is located in a sealed relationship with an outer housing. The heat exchanging body, typically called a rotor, rotates about a center shaft within the housing of the heat exchanger. A plurality of radial walls extend radially outward from the center shaft and subdivide the heat exchanging body into a plurality of angular sectors. The angular sectors have a core material to provide a path for heated exhaust or intake air to travel through. The core is heated by the exhaust and the heat energy of the core is transferred to the intake air when the heated core is exposed to the intake air. As the heat exchanging body rotates, the angular sectors are alternatively exposed to the hot and cold conduits of the heat exchanging apparatus. Hence, as an angular sector of the heat exchanging body is exposed to the hot conduit, it absorbs heat from the exhaust gases of the machinery. The sector then rotatably moves and is exposed to the intake conduit where the sector then releases heat into the cool air that is passed into the machinery intake.
One difficulty with these heat exchangers is that there is typically a gap that exists between the rotor and the inner walls of the housing. Consequently, some heated gas in the exhaust conduit, may flow through the gap between the rotor and the inner wall of the housing and thereby bypass the core material in the rotor. Similarly, cool air in the intake conduit may also flow through the bypass gap between the rotor and the inner wall of the housing similarly bypassing the core material of the rotor. It will be appreciated that the amount of gas that bypasses the core material of the rotor in both the exhaust conduit and the intake conduit reduces the efficiency of the heat exchanger as a greater amount of unheated air is being provided to the power plant and a greater amount of heated gas is being exhausted through the exhaust conduit without heating the rotor.
To address this particular problem, seals are typically installed at the upper and lower surfaces of the rotor which extend into the gap between the outer surface of the rotor and the inner surface of the housing. These seals are typically referred to as either circumferential or bypass seals and they generally extend around the entire circumference of the rotor. These seals can either be mounted on the rotor so as to extend outward against a surface on the inner wall of the housing or they can be mounted on the housing so as to extend inward towards a surface on the rotor. In effect, these seals occlude the opening between the inner wall of the housing and the outer wall of the rotor and thereby direct the hot gas in the exhaust conduit through the core material of the rotor and similarly direct the cool air in the intake conduit through the core material of the rotor.
One difficulty associated with the use of these seals is that the rotor will generally deform during operation as a result of differential thermal expansion. This deformation is typically referred to as turndown and is often exhibited by the outer surfaces of the rotor sagging downward with respect to the center axis and toward the housing. It will be appreciated that a bypass seal mounted on the outer surface of the rotor that is positioned so as to be immediately adjacent the inner wall of the housing will make contact with the housing when the rotor turns down. This may result in the seal becoming unduly worn or damaging the inner walls of the housing.
One solution to this problem is to position a seal in the bypass space so that a measured gap remains between the outer surface of the seal and the adjacent wall. The measured gap is selected so that when the rotor deforms as a result of turndown, the outer surface of the seal is positioned immediately adjacent the wall. However, positioning a seal in this manner results in the seal not being particularly efficient when the heat exchanger has not fully turned down as gases can flow through the measured gap, thereby reducing the efficiency of the heat exchanger. Further, the degree of turndown of the heat exchanger may vary or even be unknown during operation which could result in a gap remaining during operation of the heat exchanger thereby reducing the efficiency of the heat exchanger.
Another possible solution is to use a seal that is flexible so that when the heat exchanger turns down, the seal can resiliently contact the sealing surface. A flexible seal can then be positioned substantially adjacent the sealing surface prior to turndown thereby allowing the seal to substantially occlude the bypass opening over the entire range of deformation of the rotor. However, as the seal will continuously be rubbing against the sealing surface, it is generally desirable that the seal be relatively thick so as to prolong the life of the seal against damage and the continuous wear of rubbing against the sealing surface. However, increasing the thickness of the seal to prolong the life of the seal against damage and wear naturally results in a decrease of the flexibility of the seal.
One possible solution to this problem is illustrated by the prior art seal shown in FIG. 3. As shown in FIG. 3, the seal is comprised of two relatively thick seal members 100a and 100b with a plurality of slots 102 formed therein to allow the seal to conform to the curvature of the rotor of housing. The slots 102 thereby define a plurality of tabs 104. In some applications only a single seal member 100a is mounted in the heat exchanger so that the tabs 104 can make contact with a sealing surface and partially occlude the bypass gap. As the tabs 104 are not connected to each other, they have a degree of independent resilience which allows the tabs 104 to deform against the sealing surface when the heat exchanging body deforms as a result of turndown. However, the slots 102 will still allow for some heated gases in the hot gas conduit and some cold air in the cool air conduit to bypass the heat exchanging body. The Applicant has observed heat exchanger installations using these types of seals that have an approximately 6% efficiency loss that is attributable directly to the notches 102 in the seals.
In other applications, two identical seals 100 are positioned adjacent each other in the manner shown in FIG. 3. Specifically, the tabs 104 on one seal are positioned to overlap the slots 102 on the other seal. While this will reduce the efficiency loss stemming from air and exhaust gases escaping through the slots 102, the seal is increased in thickness and is therefore significantly less flexible. The loss in flexibility often requires that these types of seals to be positioned away from the sealing surface so as to account for the deformation of the heat exchanging body occurring as a result of turndown. Further, even with overlapping seals, a gap typically remains right at the sealing surface even when two seal members are overlapping. This gap results from the seals contacting the surface at an acute angle which prevents the upper seal from contacting the sealing surface without the slots in the upper seal providing an additional leakage path. Hence, while these types of seals provide some improvement to efficiency loss in heat exchangers, they still result in inefficient operation as air and gases can bypass the heat exchanging body through the bypass gap.
Consequently, there is a need for an improved seal that can be mounted in the bypass gap between the heat exchanging body and the inner wall of a housing of a heat exchanger that will reduce the inefficiencies of the heat exchanger that result from air and gases bypassing the heat exchanging body. In particular, there is a need for a seal which can be mounted in the bypass gap so as to be positioned immediately adjacent a sealing surface so as to substantially close off the bypass gap. To this end, the seal should be flexible so that when the heat exchanging body deforms as a result of turndown, the seals can be deformed by the sealing surface while simultaneously maintaining resilient contact with the sealing surface. This seal should also be configured so as to be both flexible and yet capable of withstanding a significant amount of wear as a result of continuous rubbing contact with the sealing surface.